(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

358                                ALASKAN GLACIER STUDIES
step is found in the relationship of the channels on the east and west sides of Willard Island. It is possible that when the glaciers from either side of Willard Island united, the combined ice tongue had such increased power as to enable it to erode more deeply beyond the end of the island. Northward to the mouth of Blackstone Bay the fiord bottom slope averages 80 to 85 feet to the mile.
The channel bottom between Badger Point and the southern end of Willard Island is basin shaped, with only 90 feet of water at the northern margin and 126 feet in the center. This may be a rock basin due to glacial erosion. There may also be a rock basin southwest of the point where there is a depth of 936 feet, in Blackstone Bay, near the turn just west of the cove of Tebenkof Glacier. The increased depths in the fiord north of Tebenkof Glacier may be due to increased glacial erosion resulting from the union of this glacier with the Blackstone Bay ice tongue.
The relationship of the Tebenkof Glacier cove to Blackstone Bay is that of a submerged hanging valley, the lip of the hanging valley at the bottom of the cove being at least 700 feet above the floor of Blackstone Bay.
Glacial Deposits—Above Sea Level. As is the case in the other fiords of the Prince William Sound region, glacial deposits above sea level are relatively unimportant. The largest accumulation of this kind is the terminal moraine and outwash plain near the terminus of Tebenkof Glacier, already mentioned. There are also minute terminal moraine deposits around the borders of the Ripon and Lawrence Glaciers, and there is a thin mantle of ground moraine on the fiord walls, where not too steep, and on parts of Willard Island.
Moraine Bar at Wittard Island. The only other morainic accumulation in the bay is of considerable interest. It was sketched by Grant and Higgins in 1909, who referred to it as "modified remains of two recessional moraines." This partly-submerged terminal moraine, or moraine bar, nearly joins Willard Island with the mainland on the east (Fig. 53). It projects over three-eighths of a mile southeastward from Willard Island, extending more than half way across the fiord; further east is allow, islet; and then comes a point which projects a quarter of a mile from the main and At low tide nearly the whole of the moraine bar is exposed, there being only two narrow channels across it, the easternmost only a few feet deep, the westernmost, 24 feet deep at about mid-tide. As the submerged contours show, the fiord bottom here rises about 474 feet above the bottom of the bay immediately to the north and 210 to 288 feet above the bed of the fiord to the south. The northern face of the moraine, therefore, slopes at the rate of 1900 feet to the mile, while southward toward the glacier, the slope is only 1120 feet per mile.
Some beach sand is found on portions of this moraine and on those parts of the bar that rise entirely above high tide trees and grass are growing. That th™ bar is not a mere sand spit is shown conclusively by the character of the foundation material which is especially well exposed on the portions uncovered at low tide. It consists of angular bowlders .up to 6 feet in diameter, some of them striated, which could have reached their present position only in the glacier, being too large and too numerous for an iceberg deposit with sea level as at present, The till at the surface of this terminal moraine has largely been washed away by the tidal currents, which at high tide sweep strongly across the submerged parts of the bar.
It is clear that a moraine exists here on the surface.   Whether it is (1) a thick till